THORLABS EDU-OT2 User Manual page 19

Portable Optical Tweezers Kit
Influence of the Laser Power
While the numerical aperture affects the ratio of gradient and scattering force, the laser
power directly influences the individual forces. In Equations (13)
and (15),
we see that both forces increase proportionally with the laser power ��. When the laser
power is increased, each individual force gets stronger. However, their ratio (which
essentially determines if optical tweezing ocurrs or not) remains the same, see Equation
(28).
Combining what we know about the numerical aperture and laser power, we can note that
the increase in trapping strength can be achieved through two factors:

Increase of the numerical aperture of the objective so that the gradient force
exceeds the scattering force.

Increase of the laser power, which increases the tweezer's strength linearly if the
condition ��
This also tells us that increasing the numerical aperture is the more effective way to
improve trapping than increasing the laser power. While the laser power increases the
trapping force linearly, Figure 9 shows a stronger than linear behavior above the trapping
limit.
Page 16
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Chapter 4: Principles of Optical Tweezers
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